This invention relates generally to methods of tracking fluids flowing along a flow path. In a particular aspect, the present invention relates to a method of tracking fluids flowing in a tubing located in a well and in an annulus defined outside the tubing. In another aspect, the invention relates particularly to a method of maintaining a well pressure balanced with respect to the fluids flowing through the tubing and annulus.
When a fluid is conducted along a flow path, characteristics of the fluid sometimes need to be known to determine whether the flow is occurring as planned. This is particularly true for a flow containing different fluids conducted in series, such as during the cementing of tubular casing in the bore hole of an oil or gas well. For example, an equivalent circulating density is one characteristic used to monitor an oil or gas well cementing operation.
To cement casing in a well bore, spacers, cement slurries and other single or multiple phase fluids are pumped into the casing on top of drilling fluid which was previously placed in the well. These added fluids force the previously placed and added fluids down through the casing and up through the annulus between the casing and the well bore, expelling fluid at the top of the annulus. If the weight of the added trailing fluid exceeds the weight of the leading fluid or fluids, a "free fall" condition results.
During the normal course of a cementing operation, it is desirable to keep track of where the different types of fluids are along the flow path which includes the casing and annulus. Dynamic characteristics of these fluids during flow should also be tracked or monitored to insure that the flow is occurring as desired (for example, to insure that a cement slurry is not setting up too fast). It is also desirable to keep track of the flowing fluids to determine when free fall occurs so that a pressure balancing compensation can be made at the inlet to stop the free fall and regain control of the operation.
A type of computer-implemented method has been previously used to monitor fluid flow in a cementing operation. This has included dividing the casing and annulus flow path into segments wherein each segment has a known geometry. These segments are defined by data stored in fixed arrays within the memory of the computer. These arrays have a fixed size which limits the number of segments into which the flow path can be divided. In practice, this has prevented dynamic changes from being made as the monitored flow actually occurs. This has resulted in a lack of precision in the calculated characteristics of the fluids.
The free fall phenomenon and a type of method of trying to compensate for it are likewise known; however, this has focused on monitoring inlet pressure or other aspects of the flow which we believe produce a less precise indication of free fall.
Although there are already methods related to tracking flowing fluids and free fall, there is the need for a method which gives more flexibility in tracking fluids throughout the actual time the flow is occurring so that better, more precise information can be obtained about the tracked fluids. There is particularly the need for a method adapted to track fluids through various segments within a tubing and annulus during a cementing job. This method should also be adapted for accommodating curved segments which exist in deviated wells. There is also the need for a method particularly adapted to track fluids for maintaining a pressure balance to overcome free fall tendencies such as can occur in a vertical well in which a cement slurry is pumped in on top of one or more lighter fluids.